This patent application claims the benefit of the earlier filed Chinese Patent Application serial number 201010158668.0 filed on Mar. 31, 2010.
This invention relates generally to an aqueous copolymer dispersion for the preparation of coatings, especially for elastomeric coatings or exterior coatings, which after exposure to ultraviolet radiation, yields a relatively hard surface to improve dirt pick up resistance (DPUR) from early stage to long term exposure.
By “dirt pick up resistance” herein is meant that is the ability to minimize the accumulation of material such as dirt, dust, and soot onto the surface of the coating. DPUR is a general recognized desirable characteristic of a coating film. Coatings with poor DPUR are characterized as having an unclean and darkened appearance.
Benzophenone is known to be effective at improving the early DPUR of paint films made with soft polymers, but it is less effective for improving long term DPUR. It is also easy to cause surface cracking as a result of over crosslinking the film surface when the dosing level is relatively high. And the nature of Benzophenone requires that films containing this material be exposed to UV relatively quickly, or it will not result in the full effect.
Patent application DE 4318083 provides aqueous polymer dispersions containing a mixture of hydrophilic and hydrophobic aromatic ketones, which enable the coating systems with increased UV sensitivity with a low tendency to soiling. The advantage of this invention was contributed by a combination of both hydrophilic (component B) and hydrophobic (component C) aromatic ketones. However, comprehensive properties of the dry film surface including long term exposure performance, exterior exposure performance and color retention performance in the absence of hydrophilic aromatic ketones are still desired.